Spectral skew

Computer software is used to improve spectral quality. The most widespread procedures deal with averaging and background subtraction. The averaging process is rather obvious. The intensities of ions peaks at each m/z, recorded along the analyte chromatographic peak profile, are summed in several spectra and divided by the number of spectra used. Averaging minimizes, for example, spectral skewing problems. 

Both Q and SF mass spectrometers are scanning (sequential) analyzers and multiisotope analysis can be achieved at the expense of the measurement sensitivity and precision. The sequential measurement of m/z at different points within a time-dependent concentration proble of a transient signal can result in peak distortions and quantisation errors commonly referred to as spectral skew. The alternative is TOF-MS which features the ability to produce a complete atomic mass spectrum in less than 50 xs and thus allows very brief transient signals to be recorded with high bdelity. This is especially useful in the on-line isotope ratio determination. However, a 10-fold loss in sensitivity of a TOF-ICP-MS instrument in comparison with the latest Q instruments often creates an obstacle for the wider application of TOF-ICP-MS as a detector in the CE of metallobiomolecules in biological samples. 

Another drawback of quadrupole mass spectrometers is so-called spectral skewing, which is also related to the way in which the instrument operates. The phenomenon is illustrated in Figure... 

FIGURE 3.12 Spectral skewing and its causes, (a) A chromatographic peak with the times when ions A, B, and C are sampled during a single scan, (b) A is sampled at time A its abundance is 30,000 (c) B is sampled at time B, with abundance of 110,000 (d) C is sampled at time C, with abundance of 140,000 (e) The mass spectrum recorded for the scan from time A to time C. 

The potential of the most commonly used mass spectrometers (quadrupoles and magnetic sector-field single collectors) for elemental analysis has been discussed in detail in Parts 2.1 and 2.2.1 of this chapter. However, some intrinsic limitations still remain with these sequentially scanned systems, particularly when transient or time-dependent signals (such as those produced by laser ablation (LA), electrothermal vaporisation (ETV), flow injection (FI) and chromatography) are used to analyse a large number of isotopes. These scan-based systems can measure only a single m/z at a unit of time. Hence, truly simultaneous determination of multiple isotopes, particularly when fast transient signals are analysed, is not possible without the introduction of spectral skew . ... 

Benkhedda et al demonstrated the unique performance of ICP-TOF-MS for the simultaneous multi-element detection of 22 isotopes of rare earth elements (REEs) in a El peak of about 5 s width (at 50% height) without the introduction of any spectral skew. Eigure 2.24 represents the profiles obtained by USN-ICP-TOE-MS for three replicates of 250 ptL injections of a 0.1 mg/L multi-elemental standard of REE s after FI on-line pre-concentration in a fkr (KR) pre-coated with the chelating reagent l-phenyl-3-methyl-4-benzoyl-pyrazol-5-one (PMBP). 

The multicollector device enables real simultaneous detection of isotopes, thus eliminating classical sources of uncertainty in quadrupole-based ICP-MS isotope ratio measurements due to their sequential scanning mode of operation, and plasma flicker noise. For measurement of transient signals, the problem of so-called spectral skew, as described earlier for quadrupole ICP-MS instruments, is eliminated. 

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